The present invention is related to a multi-step process for treating hydrocarbon oil by means of a refining process and a cracking process in the absence of hydrogen. More particularly, it is a combined process of low degree solvent deasphalting and delayed coking.
Solvent deasphalting is an important technology for upgrading residue. It is a physical liquid-liquid extraction process, the basic principle of which is to carry out separation according to the difference in the solubility of various components in residue in hydrocarbon solvents. Solvent mainly dissolves saturates and aromatics and removes most resin and asphaltene in residue by regulating the operation conditions and controlling the dissolving ability of the solvent, and thereby yields deasphalted oil which has high hydrogen/carbon ratio, low carbon residue, low impurity content and can meet the requirement of feed for the downstream process. A typical solvent deasphalting process comprises introducing the stock (usually vacuum residue) into the upper part of the extractor after heat exchange to attain an adequate temperature and introducing the solvent at a certain temperature from the lower part of the extractor. The two streams flow counter-currently and come into contact in the extractor. Resin and asphaltene settle in the bottom of the extractor and the deasphalted oil enters into the settling section via the pipe riser, and then the solvent in the deasphalted oil and the asphalt is recovered respectively. The solvent is recycled for reuse.
Delayed coking is a thermal processing technology which converts the residue into gas, gasoline, diesel, gas oil, and coke by deep thermal cracking. The process of the conventional delayed coking is: after preheating in the convection section of the furnace, the stock enters into the coking fractionator, and then enters into the radiation section of the furnace, wherein it is heated to 500xc2x0 C. The heated stock enters the coker drum to carry out the coking reaction and producing coking distillate, oil and vapor escape from the top of the coker drum and enter into the fractionator to be separated into dry gas, gasoline, diesel, gas oil, while coke aggregates in the coker drum. The recycle ratio in the conventional delayed coking technology generally is 0.4.
The combination of solvent deasphalting with other heavy oil processing technologies has been a subject of research. For example, EP 209225A2 discloses a process for processing residues by combining a solvent deasphalting unit and a delayed coker. This process raises the linear velocity of the unrecovered solvent in the deoiled asphalt by vaporization in the furnace tube and makes full use of the heat source of the delayed coker to recover the solvent in the solvent deasphalting unit to achieve the goal of saving energy, but does not describe the reuse of the solvent.
V. K. Patel, et al set forth in xe2x80x9cEconomic Benefits of ROSE/Fluid Coking Integration, 1997 NPRA, AM-97-50xe2x80x9d combining the solvent deasphalting ROSE technology with fluidized coking to process heavy residues at a deep degree. Meanwhile, they also suggest a scheme combined solvent deasphalting and delayed coking, i.e., coking the deoiled asphalt, but this technology has the problems that the deoiled asphalt becomes heavier and that the furnace tube is prone to coke, and so on.
U.S. Pat. No. 4,859,284 combines solvent deasphalting and coking for treating high softening point asphalt and applies a double-screw mixing reactor to the coking section. This process will encounter engineering problems in large-scale industrial production,
EP 673989A2 combines solvent deasphalting and pyrolysis and more than 50% of the obtained deasphalted oil selves as the feed of the pyrolysis unit to yield light oil products. Since the pyrolysis is restricted by coking, the conversion degree is affected.
The present invention uses a process which combines low degree solvent deasphalting and delayed coking, i.e. uses low degree deasphalted oil rather than deoiled asphalt as a part of the feed for delayed coking and thereby makes the yield of the deasphalted oil 70 wt %-95 wt % relative to the deasphalting stock. Only heavy asphalt is removed from the stock, and the soft asphalt still remains ill the feed for the delayed coker; therefore on the one hand, the coking of the furnace tube of the delayed coking is avoided and the operation period of the delayed coker is extended, on the other hand, the yield of the liquid products of delayed coking is raised. Meanwhile, the content of the impurities in coke is reduced, and the quality of the coke product is improved.
The object of the present invention is therefore to provide a combined process of low degree deasphalting and delayed coking based on the prior art.
In the steel production wherein high power electrode is used, it is necessary to use a high or ultra-high power graphite electrode being able to bear rapid changes in conditions in a short time. For the purpose of the electrodes not cracking due to a relatively large thermal stress in rapid heating or cooling, the coefficient of thermal expansion (CTE) becomes a key index of the petroleum coke product used for high power electrode. The petroleum coke used for high power electrode exhibits a needle shape in appearance after it is cracked, and has an obvious fiber structure in micrograph and has a strong anisotropy. It has a series of merits such as low CTE, and good graphitization ability. Therefore it is a skeleton material for fabricating high power electrodes and is widely applied in the fields of steel production, aerospace, etc.
The indices of the quality of the petroleum coke used for high power electrode are shown below:
Real density, g/cm3 greater than 2.12
CTE (RIPP method),xc3x9710xe2x88x926/xc2x0 C. 2.35-2.60
Sulfur content, wt % not more than 0.7
Ash, wt % not more than 0.15
The theoretical basis for the production technique of petroleum coke used for high power electrode is the formation mechanism of the mesophase and lie commonly used stock is catalytically cracking decanted oil, thermal cracking residue, extract from lube solvent refining and ethylene tar, etc. Although it is possible to produce petroleum coke used for high power electrode from the aforesaid stocks through different pretreating technologies, it is difficult to realize larger scale of production due to limited resources.
U.S. Pat. No. 4,178,229 discloses a process for producing premium petroleum coke from straight-run vacuum residue, wherein the vacuum residue first converts to distillate oil and asphalt, and the asphalt is cracked together with a hydrogen donor to produce the stock for producing premium petroleum coke. Since this patent uses a hydrocracking process to treat the residues the operation cost and expense are increased.
U.S. Pat. No. 4,130,475 discloses a process for producing premium petroleum coke from atmospheric residue, wherein the major stock is atmospheric residue and a small potion of residue for ethylene is incorporated therein. The mixture directly enter into a delayed coker without any other treatments, and the distillate oil produced in the coker subjects to thermal cracking reaction via two thermal cracking furnaces. The thermal cracking residue formed in the reaction returns to the feed inlet of the coker and mixes with the atmospheric residue. This patented technique exerts a special limit to the atmospheric residue stock since the atmospheric residue stock is not subjected to any chemical treatment, but only other stocks are added thereto and mixed. Therefore, the sources of the stock are restricted.
The present invention make the yield of the deasphalted oil 70 wt %-95 wt % by using a combined process of low degree deasphalting and delayed coking, wherein the low degree deasphalted oil rather than the deoiled asphalt serves as a part of the feed for the delayed coking. The low degree deasphalted oil is produced by only removing the asphaltene in the stock, and most of the oils and resins in the stock still remains, therefore on the one hand, coking of the furnace tube of the delayed coker is avoided and the run length of the delayed coker is extended, on the other hand, the yield of the liquid products of delayed coking is raised. Meanwhile, the sources of the stocks for producing the needle coke are enlarged and the content of the impurities in the needle coke is lowered.
Therefore, another object of the present invention is to provide a combined process of low degree solvent deasphalting and delayed coking based on the prior art to produce needle petroleum coke used for high power electrode while increase the yield of the liquid products.
The present invention provides a combined process of low degree solvent deasphalting and delayed coking, which comprises the following steps:
(1) A preheated deasphalting stock and a solvent enter into an extractor, and a solvent-containing deoiled asphalt solution is withdrawn from the bottom of the extractor. The deasphalted oil solution from the top of the extractor yields deasphalted oil after recovering the solvent. The yield of the deasphalted oil is 70 wt %-95 wt % relative to the deasphalting stock; and
(2) A part or all of the deasphalted oil and the coker recycle oil, and optionally another conventional coking stock enter into the convection section of the furnace of the delayed coker for heating, then enter into the radiant section of the furnace for heating, and finally enter into a coker drum to conduct the coking reaction. Coke remains in the coker drum, coker oil and vapor escape from the coker drum and then enter into the fractionator, wherein they are separated into gas, gasoline, diesel, and gas oil.
The present invention also provides a combined process of low degree solvent deasphalting and delayed coking, which comprises the following steps:
(1) A preheated deasphalting stock and a solvent enter into an extractor, and a solvent-containing deoiled asphalt solution is withdrawn from the bottom of the extractor. The deasphalted oil solution from the top of the extractor yields the deasphalted oil after recovering the solvent. The yield of the deasphalted oil is 70 wt %-95 wt % relative to the deasphalting stock;
(2) A part or all of the deasphalted oil and coker recycle oil enter into the furnace of a mild thermal converter, and the light components obtained by separating the heated stream enter into the fractionator of the delayed coker while the heavy components enter into the furnace of the delayed coker; and
(3) The heavy components subjected to the mild thermal reaction and optionally another needle coke stock enter into the furnace of the delayed coker for heating and then enter into the coker drum for coking reaction. Coker oil and vapor escape from the top of the coker drum and enter into the fractionator to be separated into gas, gasoline, diesel, and gas oil. The coker recycle oil is mixed with deasphalted oil and the mixture enters into the furnace of the mild thermal converter, and the needle coke remains in the coker drum.